The present invention relates to polarimeters, and more particularly, to a sample cell and sample cell holder in which a thermo electric cooler is used to control the temperature of the sample cell and the liquid contained therein.
A polarimeter is an instrument for measuring the optical activity exhibited by an optically active substance including inorganic and organic compounds. In particular, the plane of polarization of a linearly polarized light beam is rotated when passing through the optically active substance, and the rotation is determined by the specific substance, the concentration of that substance and the optical pathlength of the light through the substance contained in the sample cell, whereby the concentration of the specific substance can be calculated by the measured rotation. The optical rotation is also affected by the temperature of the substance contained in the sample cell, and thus it is desirable, and in many cases required by governing regulations, that the temperature of the sample substance be controlled to be at a set point for precise measurement.
As shown in FIG. 1(A) and (B), a sample cell 11 is usually a cylindrical tube so as to minimize the amount of the sample substance that is in the cell but not in the light path, referred to as xe2x80x9cdead volumexe2x80x9d. A pair of flanges 15 are provided at opposite ends of the sample cell 11, each with a beam aperture 18 therein for allowing the polarized light beam (shown as arrow 17) to pass therethrough. The flanges are usually standard in size for properly positioning the sample cell 11 in the light path when the pair of flanges sit on the cell holder constituted by a pair of parallel rails 16. The flanges may be removable from the sample cell 11 to facilitate cleaning of the cell. The temperature control of the sample substance is realized by circulating water in a water jacket 14 around the external surface of the sample cell 11 containing the sample substance 12. The water jacket 14 is formed by an outer tube 13 communicated with a water tube 19 connected to a water source at a predetermined temperature. However, such a water temperature control system is complicated in structure, comprising a water jacket and cooling tubes. Furthermore, it is inconvenient to connect and disconnect the tubing when cleaning the cell and changing the sample. It also takes long time to change the temperature set point.
Recently, thermoelectric temperature control techniques such as thermo electric coolers (TEC) have been introduced for temperature control in a polarimeter. The temperature of one side of a TEC device, which is usually a flat plate in shape, is controllable by an electric current. Heat can be made to flow through the device in either direction as required. As shown in FIG. 2, a TEC element 63 with a heat sink 64 is provided to be thermally conductive with a rectangular cell holder 62 accommodating a rectangular sample cell 61. A polarized light beam (shown as arrow 67) passes through the sample cell 11 via the apertures 68 provided on the cell holder 12. The solid TEC element 63 eliminates the complication and inconvenience of the water tubing required in the conventional cylindrical cell samples. Furthermore, the temperature of the cell holder 62 and therefore the sample cell 61 can be easily and quickly controlled at a set point. However, the rectangular sample cell 61 assumes a high dead volume and requires a larger sample volume to fill. This is costly when the measured substance is precious. The other problem with such a rectangular structural design is that the cell holder 62 can not work with conventional standard cylindrical sample cells which are commonly used in the industry. In addition this design was not able to control temperature within the limits required by relevant governing regulations.
An object of the invention is to provide a sample cell with a substantially cylindrical internal volume with its temperature controllable by a TEC element.
A further object of the invention is to provide a cell holder which is capable of holding both the inventive sample cell in a temperature controlled mode and conventional standard cells which are not temperature controlled by the cell holder. These conventional cells my be either of the water jacketed temperature controlled type or the non jacketed, non temperature controlled type.
A further object of the invention is to provide a cell holder for holding cylindrical sample cells with its temperature controllable by a TEC element.
A further object of the invention is to provide a cell holder with TEC temperature control unit which is capable to work with the conventional standard sample cells currently in common use.
To achieve the above objects, the sample cell of the present invention comprises a thermally conductive body enclosing a substantially cylindrical internal volume with an insulating outer cover which leaves one face exposed for complementarily contacting a temperature controlled baseplate so as to keep the sample cell at a predetermined temperature, and a means for positioning the sample cell at a predetermined position such that a measuring light beam longitudinally passes through the sample cell.
Thus, the temperature of the sample cell, which is less in dead volume, is controlled by the temperature control means. The water jacket system required for the conventional cylindrical sample cells is eliminated.
Preferably, the base plate which is fixedly positioned relative to the light path of the light beam for positioning purposes, is temperature controlled by means of being thermally conductive with a TEC element.
In a preferred embodiment, the heat transfer element is a cell mount on which the cylindrical sample cell rests. The cell mount is placed on the base plate in a predetermined position.
Preferably, the base plate comprises a slant upper surface on which the cell mount rests, and the cell holder further comprises a horizontal rail. The rail, on the one hand, stops the cell mount at a predetermined position on the base plate, and on the other hand, enables the inventive cell holder capable of supporting a conventional standard cylindrical sample cell by supporting its flanges directly resting on the rail and the slant upper surface of the base plate.
In another preferred embodiment, the heat transfer element is a cover resting on the cylindrical sample cell which sits on a pair of parallel rails. Preferably, the heat transfer element is detachably connected to the base plate so that the base plate can work with different heat transfer elements for different sizes of cylindrical sample cells.